ML20211N051
| ML20211N051 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 09/03/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20211N031 | List: |
| References | |
| NUDOCS 9909100038 | |
| Download: ML20211N051 (8) | |
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"E NUCLEAR REGULATORY COMMISSION 5 wAswiwovow, p.c. sones.aoot SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION EXEMPTION FROM REQUIREMENTS OF 10 CFR 50.44 SOUTHERN CALIFORNIA EDISON COMPANY L SAN DIEGO GAS AND ELECTRIC COMPANY THE CITY OF RIVERSIDE. CALIFORNIA
! THE CITY OF ANAHEIM. CALIFORNIA l SAN ONOFRE NUCLEAR GENERATING STATION. UNITS 2 AND 3 DOCKET NOS. 50-361 AND 50 362 l
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1.0 INTRODUCTION
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' By letter dated September 10,1998,'as supplemented July 19,1999, Southem Califomia
. Edison Company (SCE), the licensee for San Onofre Nuclear Generating Station (SONGS) -
_ Units 2 and 3, requested an exemption from certain requirements of 10 CFR 50.44, and 10 CFR Part 50 Appendix A, General Design Criterion (GDC) 41. The purpose of the exemption request was to remove requirements for hydrogen control systems from the SONGS ;
Units 2 and 3 design basis.' The proposed request for exemption included amendment application numbers 180 and 166 for Units 2 and 3, respectively, which would remove the hydrogen control system from the plant technical specifications (TSs) and the updated final
' safety analysis report (UFSAR). )
During a June 2,1999, meeting with the U.S. Nuclear Regulatory Commission's (NRC's)
' Advisory Committee on Reactor Safeguards (ACRS), the licensee stated its intent to place on hold the request for exemption from hydrogen monitoring requirements. Accordingly, this safety evaluation does not address the hydrogen monitoring requirements. As far as the hydrogen recombiners and the hydrogen purge subsystem, the licensee verbally committed to not remove them from the plant and continue to include them in the plant's severe accident management guidelines. Finally, the licensee committed to notify the staff should it abandon its attempts to continue to maintain either the hydrogen recombiners or the hydrogen purge subsystem. In its letter dated July 19,1999, the licensee docketed this commitment.
2.0 - DISCUSSION AND EVALUATION 2.1 SONGS Units 2 and 3 Hydrocen Control System
' Following an accident, a potential hazard may result from the buildup of hydrogen in the containment. Hydrogen may be produced by radiolytic decomposition of water, metal-water 9909100038 990903 '
2-reaction involving the zirconium fuel cladding and reactor coolant, and by corrosion, particularly,
' corrosion of zinc-based paint. Metal-water reaction rapidly produces a relatively large amount of hydrogen, whereas radiolysis and corrosion produce hydrogen at lower rates over an extended period of time.' If no action is taken, hydrogen could continue to accumulate until a concentration might eventually be reached at which a deflagration or detonation could occur.
The SONGS hydrogen control system consists of hydrogen recombiners, a hydrogen purge subsystem, and a hydrogen monitoring subsystem. The hydrogen control system was installed to control the hydrogen concentration inside containment below the lower flammability limit of 4 volume percent following design-basis loss of-coolant accident (LOCA) conditions. UFSAR Section 6.2.5 assumes a quantity of hydrogen prescribed by the U.S. NRC Regulatory Guide (RG) 1.7, " Control of Combustible Gas Concentrations in Containment Following a Loss-of-Coolant Accident." The hydrogen purge subsystem serves as a backup for the hydrogen recombiners and provides a means of reducing containment hydrogen concentration by venting and purging the containment atmosphere at a controlled rate.
' The hydrogen recombiners at SONGS Units 2 and 3 consist of two 100 scfm (standard cubic feet per minute) electric-powered, thermal recombiners located inside the containment. The recombiners are not operable under station blackout conditions. Operation of the recombiners i does not involve any release of effluent outside the containment. Assuming an amount of hydrogen prescribed by the RG 1.7 and no operator action to initiate the hydrogen control systems, the containment hydrogen concentration would reach 3 percent after approximately 9 days. The hydrogen recombiners are manually activated before the hydrogen concentration reaches 3 percent indicated. If neither hydrogen recombiner was operable, the hydrogen purge would be used.
- The hydrogen purge subsystem is a backup to the hydrogen recombiners. The hydrogen exhaust and supply trains consist of filters, fans, fan heaters, and associated piping, valves, ductwork, dampers, instruments, and controls. In the event the recombiners fail or cannot be used, the exhaust fans could be used to transfer combustible gases from within containment through a filter prior to release. Outside air can be supplied into containment by the system in order to dilute the hydrogen concentration. Although the hydrogen purge subsystem vent valves are designed for 60 psig, which is the containment design pressure, the associated duct work and filters are rated for much lower pressures. The subsystem can purge the containment atmosphere at a rate of 50 scfm.
Guidance on the operation of the hydrogen purge system during beyond design-basis accidents is provided in the plant's severe accident management guidelines. Although the filtered hydrogen purge system is the preferred vent path, it would most likely not be used because the ;
severe accident management guidelines do not recommend venting prior to reaching 99 psig, I which is beyond the design capacity of the hydrogen purge system. Venting at 99 psig is accomplished by using the mini-purge system which is designed for 150 psig.
2.2 Raoulatorv Reauirements For Combustible Gas Control Systems Regulatory requirements for the hydrogen control system are specified in 10 CFR 50.44 and i 10 CFR Part 50, Appendix A (GDC 41,42, and 43). Additional NRC staff guidance is provided '
in RG 1.7.. NRC staff review and acceptance criteria are specified in Standard Review Plan l
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3-(SRP) 6.2.5, " Combustible Gas Control in Containment." Different requirements apply to facilities according to the date of publication of the Notice of Hearing for the Construction Permit. With regard to hydrogen recombiner and purge-repressurization system requirements,
' SONGS Units 2 and 3 are subject to the requirements of 10 CFR 50.44(e) which states:
For facilities whose notice of hearing on the application for a construction permit was published on or after November 5,1970, purging and/or repressurization shall not be the primary means for controlling combustible gases following a LOCA. However, the capability for controlled purging shall be provided. For these facilities, the primary means for controlling combustible gases following a LOCA shall consist of a combustible gas control system, such as recombiners, that does not result in a significant release from containment.
SONGS Units 2 and 3 are also subject to 10 CFR 50.44(d) which states :
For facilities that are in compliance with [section) 50.46(b), the amount of hydrogen contributed by core metal-water reaction (percentage of fuel cladding that reacts with water), as a result of degradation, but not total failure, of emergency core cooling functioning shall be assumed either to be five times the total amount of hydrogen calculated in demonstrating compliance with [section) 50.46(b)(3), or to be the amount that would result from reaction of all the metalin the outside surfaces of the cladding cylinders surrounding the fuel (excluding the cladding surrounding the plenum volume) to a depth of 0.00023 inch (0.0058 mm), whichever amount is greater, l
The amount of hydrogen described by 10 CFR 50.44(d) was clearly an attempt to address )
accident sequences beyond the design basis. As stated in the statement of considerations, l 41 FR 46467, and RG 1.7, the factor of 5 is intended to provide an appropriate safety margin i against unpredicted events during the course of accidents. More specifically,it is to account for !
a more degraded condition of the reactor than the emergency core cooling system (ECCS) l design basis permits. RG 1.7 assumes oxidation o! up to 5 percent of the zircalloy surrounding the active fuel. For calculating the amount of hydrogen riue to radiolysis recommended by RG 1.7, the fission product distribution modelincludes: 50 percent of the halogens,1 percent of the solids present in the core which are intimately mixed with the coolant water, all noble gases released to the containment, and all other fission products that remain in the fuel rods.
Subsequent risk studies have shown that the majority of risk to the public is from accident sequences that lead to containment failure or bypass, and that the contribution to risk from j accident sequences involving hydrogen combustion is actually quite small. This is true despite ,
the fact that the hydrogen produced in these events is substantially larger than the hydrogen production postulated by 10 CFR 50.44(d) and RG 1.7. Hydrogen combustion sequences that could lead to early* containment failure typically involve up to 75 percent core metal-water reaction. Hydrogen combustion sequences that could lead to late containment failure involve ;
greater amounts of hydrogen due to the interaction of corium and the concrete basemat after vessel breach. Although the recombiners are effective in maintaining the RG 1.7 hydrogen concentration below the lower flammability limit of 4 volume percent, they are overwhelmed by the larger quantitles of hydrogen associated with severe accidents which are typically released over a much shorter time period (e.g.,2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />).
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d The NRC staff investigated the risk from hydrogen combustion as part of NUREG-1150.
' Because the Zion containment was found to be quite strong by the structural experts who considered the issue, early containment failure due to hydrogen bums was not modeled for Zion. Figure 7.3 of NUREG-1150, Volume 1, dated December 1990, displays information in
' which the conditional probabilities of four accident progression bins, e.g., early containment
. failure, are presented for the Zion plant, which has a large, dry containment similar to SONGS Units 2 and 3. This information indicates that, on a plant damage state frequency-weighted.
average, the mean conditional probabilities from intemal events of (1) early containment failure
. from a combination of in-vessel steam explosions, overpressurization, and containment isolation failures is 0.014, (2) late containment failure, mainly from basemat melt-through, is 0.24, (3) containment bypass from interfacing-system LOCA and induced steam generator tube rupture is
- 0.006, and (4) probability of no containment failure is 0.73. The accident progression event trees used to generate these bins are described in NUREG/CR-4551," Evaluation of Severe
- Accident Risks: Methodology for the Containment, Source Term, Consequence, and Risk Integration Analyses," Volume 7, Revision 1, Part 1. NUREG/CR-4551 goes on to state that hydrogen combustion in the period before vessel failure is now generally considered to present no threat to large, dry containments. Table A.4-5 of NUREGICR-4551 shows that the contribution of hydrogen combustion to late containment failure is also very small (only 0.5 percent of the late containment failure bin,8.376E-4, is from hydrogen combustion). Although the modeling c'f the accident progression event trees may have changed since 1990, the relative importance of hydrogen combustion for large, dry containments with respect to containment failure has not changed and continues to be quite icw.
The SONGS Units 2 and 3 Individual Plant Examination (IPE) also found containment failure due to hydrogen combustion unlikely. The IPE concluded that for the worst-case accident sequence with respect to hydrogen combustion, it is unlikely that enough hydrogen would accumulate to produce a hydrogen bum that could challenge the containment ultimate pressure capacity, The worst-case scenario was based on a hypothetical hydrogen concentration of 11.5 percent that corresponds to approximately 75 percent core metal-water reaction.
Although hydrogen igniter systems would provide added confidence that containment integrity can be maintained during hydrogen bums, Generic issue (GI)-121, " Hydrogen Control for PWR
[ pressurized-water reactor] Dry Containments," found that hydrogen combustion was not a significant threat to dry containments and concluded there was no basis for new generic hydrogen control measures (i.e., igniters).
From this information, the NRC staff concludes that the quantity of hydrogen, prescribed by 10 CFR 50.44(d) and RG 1.7, which necessitates the need for hydrogen recombiners and its backup the hydrogen purge system is bounded by the hydrogen generated during a severe accident. The NRC staff finds that the relative importance of hydrogen combustion for large, dry containments with respect to containment failure to be quite low. This finding supports the argument that the hydrogen recombiners are insignificant from a containment integrity
. perspective.
' Although not modeled in present risk studies, the hydrogen recombiners are of some value from a severe accident management perspective by preventing the uncontrolled burning of hydrogen due to radiolytic decomposition of water and corrosion in the long term. The NRC staff will discuss the worth of the recombiners from this perspective in Section 2.3 of this report.
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n GDC 41,' 42, and 43 require that a containment atmosphere cleanup system be provided to control hydrogen concentration, and the system be designed to permit inspection and testing.
. The regulation at 10 CFR 50.36 requires that appropriate operability and surveillance be
- included in the facility TSs.
2.3 Analysis Analyses performed by the NRC staff and licensee using more realistic assumptions than those used for the design-basis case, show that the lower flammability limit is not exceeded for 30 days. This is not to imply that the intent of the rule was to prevent the lower flammability limit from being exceeded for 30 days. However,30 days represents a more than sufficient time period in which ad hoc measures might be effectively implemented and would allow for decay of the fission product source term should containment venting be deemed necessary, in order to gain an understanding of hydrogen production under more realistic design-basis l
conditions than those assumed in RG 1.7, the licensee utilized the hydrogen generation rates described in UFSAR Sections 6.2.5.3.A.1 (radiolysis) and 6.2.5.3.A.2 (metal-water reaction) with the following modifications: (1) for the amount of hydrogen generated by radiolysis in the sump, the licensoe removed the 20-percent conservatism in the isotope energy production rates, (2) for the amount of hydrogen generated by radiolysis in the sump, the licensee modified the hydrogen yield from 0.5 molecule /100 ev to 0.4 molecule /100 ev, and (3) for the amount of hydrogen generated by the metal-water reaction prescribed by 10 CFR 50.44(d)(1), the licensee used the 0.23 mil cladding penetration depth criterion rather than the factor of 5 increase used in the UFSAR Section 6.2.5.3.A.2.: The analysis showed that the hydrogen concentration will
- not exceed 4 percent at 30 days post-LOCA without operation of any hydrogen control system.
As mentioned in the previous section, the risk associated with hydrogen combustion is not from design-basis accidents but from severe accidents. The hydrogen recombiners are
. overwhelmed by the metal-water reaction and are incapable of removing appreciable amounts of hydrogen in the time period prior to spurious ignition. The recombiners are, however,
' capable of preventing the uncontrolled buming of hydrogen due to radiolytic decomposition of water and corrosion in the long term.
The NRC staff performed additional analyses to ascertain the value of the hydrogen recombiners in preventing the uncontrolled burning of hydrogen in the long term under best-estimate severe accident conditions versus the previous design-basis case. The NRC staff used its confirmatory code COGAP to estimate the amount of hydrogen due to radiolytic decomposition of water and corrosion. COGAP was developed by the NRC staff for determining hydrogen concentrations within reactor containments following a design-basis LOCA. The following are some of the input assumptions the NRC staff changed to make the
- calculations more appropriate for a best-estimate severe accident analysis: (1) the amount of solid fission product decay energy absorbed by the sump water solution was increased from 1
- percent to 8 percent, (2) the iodine isotope decay energy absorbed by the sump water solution was increased from 50 percent to 75 percent, (3) the hydrogen yield was reduced from 0.5 molecule /100 ev to 0.4 molecule /100 ev, and (4) best-estimate corrosion rates. The amount of solid fission product and iodine isotope decay energy were based on the release fractions in NUREG-1465, " Accident Source Terms for Light-Water Nuclear Power Plants," and the decay
v-energy in NUREG/CR-4169,"An Approach To Treating Radionuclide Decay Heating for Use in
- the MELCOR Code System." The corrosion rates were based on the proceedings of the Second intemational Conference on the impact of Hydrogen on Water Reactor Safety,
- Albuquerque, New Mexico, October 1982. The analysis calculated the hydrogen concentration to be 5.4 percent at 30 days and did not exceed the lower flammability limit of 4 percent for 16 days. In the event that the installed recombiners were not available, ad hoc measures to obtain
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and install an extemal recombiner could reasonably be expected to be completed within this time period.
As discussed earlier, the hydrogen concentration calculated is clearly bounded by hydrogen
- generated during a severe accident and would not be a threat to containment integrity. Such a bum would impose a temperature transient to available instrumentation and equipment. In the range of 4 to 6 percent, the temperature transient is fairly benign because the rate of flame propagation is less than the rate of rise of the flammable mixture. Therefore, the flame can propagate upward, but not horizontally or downward. In this case, complete combustion will not occur until the concentration is increased above 6 percent.
Equipment survivability in concentrations greater than 6 percent was addressed as part of GI-121 (Generic issue 121, " Hydrogen Control For Large, Dry PWR Containments") which references NUREG/CR-5662, " Hydrogen Combustion, Control, and Value-impact Analysis for 1
PWR Dry Containment," which assessed the benefits of hydrogen igniters. NUREG/CR-5662 concluded that simulated equipment can withstand a LOCA and single bum resulting from a 75-percent metal-water reaction in a large, dry containment. However, the multiple containment bums due to the operation of ignition systems could pose a serious threat to safety-related equipment located in the source compartment. The multiple bum environment was found to be a threat because the source compartment temperature remains elevated from the previous bum. This is not a concern for the above radiolysis case because the containment temperature prior to the burn would be low, between 90 and 100 *F, and the time between bums would allow for the dissipation of heat from the bum. Therefore, an additioaal burn in the long term due to radiolysis and corrosion would not have a similar impact on equipment survivability.
2.4 Risk Reduction Due to Instruction Simolification
' In a postulated LOCA, the SONGS Units 2 and 3 Emergency Operating instructions (EOls) direct the control room operators to monitor and control the hydrogen concentration inside the containment after they have carried out the steps to maintain and control the higher priority critical safety functions. The key operator actions in controlling the hydrogen concentration are to place the hydrogen recombiners or hydrogen purge system in operation which involves many procedural steps. These hydrogen control activities could distract operators from more important tasks in the early phases of accident mitigation and could have a negative impact on the higher priority critical operator achons. An exempton from hydrogen recombiner and purge-repressurization system requirements will eliminate the need for these systems in the EOls and hence simplify the EOls. As mentioned previously, the NRC staff still expects the licensee's
.~ severe accident management guidelines to address the removal of hydrogen due to radiolysis in the long term. The NRC staff concludes that this simplification would be a safety benefit and, therefore, is acceptable.
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3.0 CONCLUSION
The NRC staff has established certain criteria that permit any interested person to request specific exemptions to its rules and regulations provided special circumstances exist. Special circumstances are identified in 10 CFR 50.12(a)(2). The regulation at 10 CFR 50.12(a)(2)(ii) states, " Application of the regulation in the particular circumstances would not serve the underlying purpose of the rule or is not necessary to achieve the underlying purpose of the rule." The underlying purpose of 10 CFR 50.44 is to show that following a LOCA an uncontrolled hydrogen-oxygen recombination would not take place, or that the plant could withstand the consequences of uncontrolled hydrogen-oxygen recombination without loss of safety function. Based on the above, the licensee has successfully demonstrated that the plant i could withstand the consequences of uncontrolled hydrogen-oxygen recombination without loss of safety function without credit for the hydrogen recombiners or the hydrogen purge system for not only the design-basis case but the more limiting severe accident with up to 75 percent metal-water o hon that remains in-vessel. Therefore, the NRC staff believes that the requireme~ 4 hydrogen recombiners and the backup hydrogen purge capability as part of the ,
design-bam for large, dry containments, such as SONGS Units 2 and 3, are unnecessary and I their removal from the design-basis is justified. .
The basis for this conclusion is that numerous risk studies, such as NUREG-1150 and those performed by the licensee, have shown that the relative importance of hydrogen combustion for ;
large, dry containments with respect to containment failure to be quite low. The hydrogen l recombiners are overwhelmed by the metal water reaction and are incapable of removing appreciable amounts of hydrogen in the time period prior to spurious ignition. The plant's severe accident management guidelines advise against venting prior to reaching 99 psig which is beyond the design capability of the hydrogen purge subsystem. Equipment survivability from l a hydrogen bum was addressed during the resolution of Gl-121. Elimination of the hydrogen recombiners and the hydrogen purge subsystem from the EOls would be a simplification and a safety benefit.
The recombiners and the purge subsystem would be of value in the long term by preventing a l subsequent burn due to radiolysis and corrosion. The NRC staff still expects the licensee's severe accident management guidelines to address the removal of hydrogen due to radiolysis and corrosion in the long term. In its letter of July 19,1999, the licensee committed to not remove the hydrogen recombiners and the hydrogen purge subsystem from the plant and continue to include them in the plant's severe accident management guidelines. Should the licensee decide to discontinue to maintain either the hydrogen recombiners or the hydrogen purge subsystem, the licensee would inform the NRC staff prior to implementing its action.
By application dated October 15,1993, the licensee for the Waterford 3 Steam Electric Station requested an exemption to 10 CFR 50.44 and eliminate operability and surveillance l requirements associated with the hydrogen recombiners. The NRC staff rejected the licensee's l application until such time as the licensee demonstrated that radiological consequences criteria can be met without reliance on recombiners. The overall public risk and radiological consequences from reactor accidents is dominated by the more severe core damage accidents that involve containment failure or bypass. As discussed above, SCE has successfully i-
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. .a i f demonstrated that hydrogen recombiners are insignificant from a large, dry containment integrity perspective and the radiological consequences remain unchanged with or without recombiners. Therefore, this stipulation has been satisfied for this request.
During the 463'8 meeting of the ACRS, June 2-4,1999, the Committee reviewed the request by SCE for a bcense exemption to the hydrogen control requirements for SONGS Units 2 and 3.
In a June 9,1999, memorandum, the Committee stated it had no objection to the NRC staff's approving this license exemption request, as modified to maintain the requirements for ,
containment hydrogen monitoring capability. j Principal Contributor: M. Snodderly Date: September 3,1999 1
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